1. Field of the Invention
The present invention relates to a broadcast receiver for receiving airwaves of a predetermined channel sent from plural broadcast stations by a directional switching antenna.
2. Description of the Related Art
In Japan, airwaves of television broadcasts etc. are generally sent from a base station of one place. As a result of that, almost all the airwaves sent by plural broadcast stations can be received by directing an antenna for broadcast receiving to the base station and fixing the antenna to a roof or a veranda, etc. of a house and connecting the antenna to a broadcast receiver through a cable. On the other hand, for example, in the U.S.A., airwaves are individually sent from plural broadcast stations, respectively. As a result of that, when an antenna is directed at one place and is fixed, airwaves of the broadcast station located in a direction in which the antenna is directed can be received, but airwaves of the broadcast station located in a direction in which the antenna is not directed cannot be received.
Therefore, it is necessary to switch directivity of an antenna in plural directions in order to receive radio waves from plural broadcast stations located in different bearings by one antenna. An antenna capable of switching such directivity (hereinafter called “a directional switching antenna”) is described in, for example, JP-A-2001-168627 (Paragraph 0015, FIG. 1) and JP-A-54-16155 (Page 2, Line 1 of the upper left field to Page 3, Line 12 of the upper right field, FIGS. 1 to 5) mentioned below. The directional switching antenna is constructed of plural antenna elements and phase shifters, etc. corresponding to each of the antenna elements, and directivity is electrically varied by adjusting phases of signals received in each of the antenna elements by each of the phase shifters and combining the phases. For example, in a smart antenna which is one example of the directional switching antenna, it is stipulated that directivity should be switched in 16 directions.
In the case of using such a directional switching antenna, airwaves from each of the broadcast stations can be received by sending a control signal from a broadcast receiver to the antenna and switching directivity of the antenna in a predetermined direction. As a result of this, a user can receive almost all the airwaves sent from plural broadcast stations interspersed even when the directional switching antenna is fixed and attached to a roof etc. of a house.
In the broadcast receiver for receiving airwaves by the directional switching antenna as described above, when a lock state of a frequency deviates, a reception impossibility state in which the airwaves received until then cannot be received normally occurs. In such a case, signal return processing for returning the state to a normal reception state is required. As a method of the signal return processing, for example, it is contemplated to sequentially scan all the bearings of the antenna and retrieve a receivable direction. However, in this method, possibility or impossibility of reception for all the directions is searched, so that accuracy of retrieval is high, but it often takes time to perform the signal return and in all the cases, the method is not necessarily an optimum method.
Therefore, an object of the invention is to provide a broadcast receiver capable of efficiently detecting a receivable direction and reducing time necessary for a signal is return when impossibility of reception occurs in a directional switching antenna.